Three-dimensional component integration technology (3D-integration) requires the transfer of one or more layers of microcomponents onto a final substrate, the final substrate itself possibly incorporating microcomponents. The transferred layer or layers include microcomponents (electronic, optoelectronic, etc., components) produced at least in part on an initial substrate, the layers then being stacked on a final substrate. Primarily because of the very small size and large numbers of microcomponents present on a single layer, each transferred layer must be positioned on the final substrate with great accuracy so that they are very closely aligned with the subjacent layer. Further, it may be necessary to carry out treatments on the layer after its transfer, for example, to form other microcomponents, in order to expose the microcomponents on the surface, to produce interconnections, etc.; the treatments also have to be carried out with great accuracy in regard to the components present in the layer.
However, the applicants have discerned that following transfer, there are circumstances where it is very difficult, if not impossible, to form additional microcomponents that are aligned with all of the microcomponents formed before the transfer.
This phenomenon of misalignment is described in relation to FIGS. 1A to 1F, which illustrate an example of the production of a three-dimensional structure by transfer of a layer of microcomponents formed on an initial substrate onto a final substrate followed by the formation of an additional layer of microcomponents on the exposed face of the initial substrate after bonding. FIG. 1A illustrates an initial substrate 10 that has its own shape or “inherent geometry.” In fact, as represented in a deliberately exaggerated manner in FIG. 1A and using a grid (dotted lines) to make the deformation zones visible, the initial substrate 10 is constituted by a wafer of semiconductor material that has deformations on the micrometric scale that principally correspond to a bow and to a warp or buckle. The bow of a wafer characterizes the concave or convex deformation of the wafer as a measurement of the position from the median surface to the center of the wafer, while the warp characterizes the deformations corresponding to the differences between the maximum distance and the minimum distance of the median surface relative to a reference plane, over the whole of the median surface of the wafer. These two types of deformation mean that the inherent geometry of a wafer can be characterized; for simplification, this can be classified as chip type geometry.
As can be seen in FIGS. 1B and 1C, a first series of microcomponents 11 is formed on the surface of the initial substrate 10. The microcomponents 11 are defined by photolithography using a mask that can define the zones for forming patterns corresponding to the microcomponents 11 to be produced. As the microcomponents 11 are being defined by photolithography, the initial substrate 10 is held on a substrate carrier device 12. The substrate carrier device comprises a support platen 12a on which the initial substrate 10 is held flush, for example, by means of an electrostatic system or a suction system associated with the support platen 12a. The substrate carrier device 12 can hold the initial substrate 10 in a “stiffened” position, i.e., in a position in which bow/warp type deformations of the initial substrate 10 are reduced compared with those presented by the same substrate when it is not held by the device 12. In other words, the microcomponents 11 are formed on a substrate that is initially slightly stressed (under tension or in compression), the stresses being relaxed once the substrate is freed from the device 12. The level of that stress is also linked to the temperature to which the substrate is subjected during the step of defining the microcomponents, that temperature possibly being the ambient temperature of the environment, or a controlled temperature imposed by the substrate carrier device.
As can be seen in FIG. 1D, the face of the initial substrate 10 comprising the microcomponents 11 is then brought into intimate contact with one face of a final substrate 20. Bonding between the initial substrate 10 and the final substrate 20 is carried out, for example, and preferably by wafer bonding. A buried layer of microcomponents 11 at the bonding interface between the substrates 10 and 20 is thus obtained. After bonding and as can be seen in FIG. 1E, the initial substrate 10 is thinned in order to withdraw a portion of the material present above the layer of microcomponents 11. Thus, a composite structure 30 is obtained formed by the final substrate 20 and a layer 10a corresponding to the remaining portion of the initial substrate 10.
Once it has been bonded to the final substrate 20, the geometry of the initial substrate 10 is different from that which it had initially in FIG. 1A. This new geometry of the initial substrate 10 after bonding results, in particular, from the fact that the final substrate 20 also has an inherent geometry with bow/warp deformations that differ from those originally presented by the initial substrate 10. As a consequence, when the initial substrate 10 is brought into intimate contact with the final substrate 20, the initial substrate 10 and the final substrate 20 have to adapt, at least in part, to each others' geometries, which creates zones of tensile and compressive stresses in each of the initial 10 and final 20 substrates. When they relax, these stresses cause a modification in the geometry of the initial substrate, i.e., a modification of its original bow/warp type deformations.
This modification of the geometry of the initial substrate 10 is even more pronounced after it has been thinned (FIG. 1E). Once thinned, the thickness of the remaining portion of the initial substrate 10, corresponding to the layer 10a, is much smaller than that of the final substrate 20 that then “imposes” its geometry to a greater extent on the structure as a whole. The layer 10a must then conform to the geometry of the final substrate 20 and thus further deviate from the starting geometry of the initial substrate 10.
As can be seen in FIG. 1F, the next step in producing the three-dimensional structure consists in forming a second layer of microcomponents 12 on the exposed surface of the thinned initial substrate 10. In order to define the microcomponents 12 in alignment with the buried microcomponents 11, a photolithography mask similar to that used to form the microcomponents 11 is used. The transferred layers, like the layer 10a, typically include marks, both at the level of the microcomponents and at the level of the wafer forming the layer, which are used by the positioning and aligning tools during the technical treatment steps that are carried out during photolithography.
However, even when positioning tools are used, offsets occur between some of the microcomponents 11 and 12, such as the offsets Δ11, Δ44, or Δ88 indicated in FIG. 1F (respectively corresponding to the offsets observed between the pairs of microcomponents 111/121, 114/124 and 118/128). As when forming the microcomponents 11, the composite structure 30 formed by the final substrate 20 and the layer 10a is likewise held flush against a support platen 13a of a substrate carrier device 13 that is identical to the device 12. The zones of stress (tension and compression) imposed on the composite structure 30 and, in particular, on the layer 10a are at least in part different from those present during the formation of the microcomponents 11, since the layer 10a has a geometry in terms of bow/warp deformations that is different from that presented by the substrate 10 before bonding and thinning. As a consequence, this results in a phenomenon of misalignment (also known as overlay) between the two layers of microcomponents 11 and 12 that may be the source of short circuits or connection faults between the microcomponents of the two layers. That phenomenon of overlay thus results in a reduction in the quality and value of the multilayer semiconductor wafers that are fabricated. The impact of this phenomenon is becoming greater because of the constant demand for increasing the miniaturization of microcomponents and for increasing their integration density per layer.
Photolithography tools include algorithms for correcting certain modes of overlay (rotation, translation, etc.) that may be applied to attempt to minimize the overlay between two steps of defining or forming the components. However, it has been observed that this misalignment is not homogeneous (i.e., cannot be reduced to elementary transformations); thus, it is not possible to correct the photolithography exposure in a general and satisfactory manner in order to obtain, for each exposed field of the wafer, a satisfactory maximum value for the overlay (for example, less than 100 nm [nanometer] or 50 nm). A correction of the parameters governing the lithography exposure for each field of the wafer is not industrially desirable, and so it is important to seek to optimize the set of parameters that might lead to overlay.
Further, when a layer of microcomponents is transferred onto a final substrate having a first layer of microcomponents, it is very important to be able to minimize the overlay between the microcomponents of each of the layers when they are to be interconnected. Under such circumstances, it is not possible to compensate for overlays existing between the microcomponents of the two layers by lithography.